Method of coating with lead-free electrodeposition coating composition and coated article

ABSTRACT

The present invention relates to a cationic electrodeposition coating composition substantially free of lead compounds, and more specifically to a method of coating an alloyed hot-dip galvanized steel plate with the composition and a coated article obtained by the method having edge portions and general surfaces excellent in corrosion resistance. The invention provides a method of coating an alloyed hotdip galvanized steel plate with a lead-free cationic electrodeposition coating composition containing a rust preventive pigment to form an electrodeposition coating film excellent in corrosion resistance, and a coated article obtained by the electrodeposition.

The present application is a national phase application of InternationalApplication No. PCT/JP2004/002682 filed on Mar. 3, 2004, and claimspriority from such International application pursuant to 35 U.S.C. §365. In addition, the present application claims priority from JapaneseApplication No. 2003-059026 filed on Mar. 5, 2003. The entiredisclosures of the above-identified International and Japaneseapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a cationic electrodeposition coatingcomposition substantially free of lead compounds, and more specificallyto a method of coating an alloyed hot-dip galvanized steel plate withthe composition, capable of forming edge portions and general surfacesexcellent in corrosion resistance, and a coated article obtained by themethod.

BACKGROUND ART

Cationic electrodeposition coating has been used for undercoating bodiesand parts of automobiles, and required to form coating films with highcorrosion resistance. However, lead compounds, which have been used ashighly corrosion-resistant rust preventive pigments, are not preferredfrom an environmental point of view. Consequently, various non- orless-toxic, lead-free, rust preventive pigments have been developed anddescribed in JP-A-2000-309730, etc. Proposed in JP-A-2000-309730 is acationic electrodeposition coating composition containing a basic zincsilicate having a zinc/silicon mole ratio of 1.1/1 to 2.5/1 as the rustpreventive pigment.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method of coating analloyed hot-dip galvanized steel plate with a lead-freeelectrodeposition coating composition containing a rust preventivepigment to form an electrodeposition coating film excellent in corrosionresistance, and a coated article obtained by electrodeposition coating.

As a result of intense research in view of the above object, theinvention has been accomplished by the following embodiments.

Thus, a method of the invention is for coating an alloyed hot-dipgalvanized steel plate with a lead-free cationic electrodepositioncoating composition including a binder resin and a plurality ofpigments, and comprises the step of electrodepositing the lead-freecationic electrodeposition coating composition onto the alloyed hot-dipgalvanized steel plate, wherein the binder resin includes a cationicbase resin and a crosslinking agent, and the pigments include 5 to 30%by mass of an aluminum phosphate pigment and 5 to 30% by mass of asilicon oxide pigment having a pore volume of 0.44 to 1.8 ml/g and anaverage particle size of 10 μm or less. The lead-free cationicelectrodeposition coating composition may further include 0.1 to 10% bymass of a monoalkyltin compound based on the mass of the solid contentsof the binder resin.

A coated article of the invention is an article such as an automobilebody comprising the alloyed hot-dip galvanized steel plate coated withthe lead-free cationic electrodeposition coating film by the abovemethod. The coated article may comprise a multilayer film, which has anundercoat, an intermediate coat, an overcoat, etc. on the lead-freecationic electrodeposition coating film if necessary.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail below.

In the lead-free cationic electrodeposition coating composition used inthe method of the invention, the term “lead-free” means that thecomposition substantially contains no lead (including lead in leadcompounds). The detection limit of lead has been lowered year after yearby advancement of analytical instruments, and in the invention, the term“lead-free” specifically means that the lead content of the compositionis 10 ppm or less.

The aluminum phosphate pigment is added to the lead-free cationicelectrodeposition coating composition used in the method of theinvention. Examples of the aluminum phosphate pigments include aluminumdihydrogen tripolyphosphate, aluminum metaphosphate, and aluminumpyrophosphate. The aluminum phosphate pigments may be used alone orcombined with each other. The mass ratio of the aluminum phosphatepigment to all the pigments in the composition is 5 to 30% by mass,preferably 10 to 20% by mass. When the mass ratio is less than 5% bymass, the composition cannot show a sufficient rust preventive property.On the other hand, when the mass ratio is more than 30% by mass, thesmoothness of the electrodeposition coating film is reduced.

In the invention, the silicon oxide pigment is used as a rust preventivepigment in combination with the aluminum phosphate pigment. The porevolume of the silicon oxide pigment is 0.44 to 1.8 ml/g, preferably 0.7to 1.6 ml/g, and the average particle size of the silicon oxide pigmentis 10 μm or less, preferably 1 to 8 μm. The silicon oxide pigment may beany fine particles that comprise silicon dioxide as a main component andhave the pore volume and the average particle size within the aboveranges, such as fine particles of colloidal silica or fumed silica. Whenthe pore volume of the silicon oxide pigment is less than 0.44 ml/g, thelead-free cationic electrodeposition coating composition cannot show asufficient corrosion resistance. On the other hand, when the pore volumeis more than 1.8 ml/g, the stability of the composition is reduced.Further, when the average particle size is more than 10 μm theappearance of the electrodeposition coating film is deteriorated.

The mass ratio of the silicon oxide pigment to all the pigments in thelead-free cationic electrodeposition coating composition is 5 to 30% bymass, preferably 10 to 20% by mass. When the mass ratio is less than 5%by mass, the composition cannot show a sufficient rust preventiveproperty occasionally. On the other hand, when the mass ratio is morethan 30% by mass, there is a case where the smoothness of theelectrodeposition coating film is reduced.

The lead-free cationic electrodeposition coating composition used in themethod Of the invention preferably comprises a monoalkyltin compound asa curing catalyst (or a dissociation catalyst for a blockedpolyisocyanate compound). The monoalkyltin compound is low involatility, and thereby hardly transferred into an upper coating film inthe case where the electrodeposition coating film of the composition iscoated wet-on-wet with a chipping primer or an intermediate coatingafter the electrodeposition coating and then baked, or in the case wherea chipping primer or an intermediate coating is applied and baked afterbaking the electrodeposition coating film. Thus, the formedelectrodeposition coating film using the monoalkyltin compound isexcellent in adhesion to an upper coating film, and hardly deterioratesthe surface of the upper coating film to show excellent filmperformances because the catalyst is not volatile. The monoalkyltincompound is preferably monobutyltin oxide though there are no particularrestrictions thereon. Examples of the monoalkyltin compounds includemonobutyltin trioctanoate, monobutyltin triacetate, monobutyltintribenzoate, monobutyltin trioctylate, monobutyltin trilaurate,monobutyltin trimyristate, monomethyltin triformate, monomethyltintriacetate, monomethyltin trioctylate, monooctyltin triacetate,monooctyltin trioctylate, monooctyltin trilaurate, monolauryltintriacetate, monolauryltin trioctylate, and monolauryltin trilaurate.These monoalkyltin compounds may be used singly or in combination of 2or more compounds. Further, other dissociation catalysts to behereinafter described may be used with the monoalkyltin compounds aslong as they do not reduce the advantageous effects of the invention.

A method for introducing the monobutyltin compound into the lead-freecationic electrodeposition coating composition is not particularlyrestricted. In the case where the monobutyltin compound is solid, themonobutyltin compound is preferably dispersed using a dispersing resinas well as the pigments. Further, in the case where the monobutyltincompound is liquid, it may be dissolved in or mixed with the binderresin and introduced as a part of the binder component.

The mass ratio of the monobutyltin compound to the solid contents of thebinder resin is preferably 0.1 to 10% by mass, more preferably 0.5 to7.0% by mass. When the mass ratio of the monobutyltin compound is lessthan 0.1% by mass, the monobutyltin compound is occasionallyinsufficient in catalytic activity. On the other hand, when the massratio is increased to more than 10% by mass, the catalytic activitycannot be correspondingly improved, and further, there is a case wherethe electrodeposition coating film is decomposed by over baking.

The lead-free cationic electrodeposition coating composition used in themethod of the invention may contain zinc ions in addition to themonobutyltin compound to increase the curing efficiency. An inorganiczinc compound such as zinc oxide and zinc hydroxide or an organic zinccompound may be used to maintain the concentration of the zinc ions atthe predetermined degree. The zinc compounds may be used alone or incombination of 2 or more compounds.

Examples of the organic zinc compounds include zinc salts of organicmono- or di-acids such as formic acid, acetic acid, butyric acid,caproic acid, octylic acid, lauric acid, myristic acid, palmitic acid,stearic acid, behenic acid, neodecanoic acid, acrylic acid, crotonicacid, isocrotonic acid, undecylenic acid, oleic acid, erucic acid,sorbic acid, linolic acid, linolenic acid, bisphenylacetic acid,bisphenylbutyric acid, bisphenylpropionic acid, biscyclopentanecarboxylic acid, bisacetoacetic acid, benzoic acid, methylbenzoic acid,bismethoxybenzoic acid, bis(tert-butyl)benzoic acid, bishydroxybenzoicacid, phthalic anhydride, terephthalic acid, succinic acid, maleic acid,maleic anhydride, and fumaric acid.

The zinc ion content of the lead-free cationic electrodeposition coatingcomposition is preferably 100 to 2,000 ppm, more preferably 300 to 1,000ppm. When the zincion content is less than 100 ppm, there is a casewhere the dissociation catalyst to be hereinafter described cannot showsufficient activity, resulting in insufficient curing of theelectrodeposition coating film. On the other hand, when the zinc ioncontent is more than 2,000 ppm, the electrodeposition coating filmappearance and the electrodeposition coating workability areoccasionally deteriorated.

The cationic base resin is preferably such that the electrodepositioncoating film has high corrosion resistance. Examples of such cationicbase resins include amino-epoxy resins, amino group-containing acrylicresins, amino group-containing polyester resins, etc. Among them,preferred are amino-epoxy resins. The amino-epoxy resins can be obtainedsuch that epoxy rings of an epoxy resin is opened by an amine compoundsuch as an acid salt of a primary, secondary or tertiary amine, andcationized.

The epoxy resin used as a starting material for the cationic base resinmay be a polyphenol-polyglycidyl ether-type epoxy resin, which is aproduct of a reaction between epichlorohydrin and a polycyclic phenolcompound such as bisphenol A, bisphenol F, bisphenol S, phenol novolac,and cresol novolac, or a modified epoxy resin having an oxazolidonering. The epoxy resin is preferably the modified epoxy resin having anoxazolidone ring. The modified epoxy resin can be obtained by adealcoholization reaction of an epoxy resin with a bisurethane compoundprepared by a reaction between a diisocyanate compound and one activehydrogen compound or with a heterourethane compound prepared by areaction between a diisocyanate compound and 2 or more active hydrogencompounds. In the case of using the modified epoxy resin having anoxazolidone ring as the base resin, the electrodeposition coating filmhas excellent corrosion resistance.

The amine value of the cationic base resin is preferably 30 to 130, morepreferably 40 to 80, and the number average molecular weight ispreferably 1,000 to 20,000. When the amine value is less than 30, it isdifficult to emulsify the cationic base resin. When the amine value ismore than 130, there is a fear that the electric conductivity of theresin is increased, thereby reducing the gas pin property, reducing theCoulomb efficiency, or being disadvantageous in the electrodepositioncoating workability including re-dissolubility, etc.

Examples of acids for neutralizing the cationic base resin includewater-soluble organic acids such as formic acid, acetic acid, propionicacid, lactic acid, citric acid, malic acid, tartaric acid, and acrylicacid; and inorganic acids such as hydrochloric acid, phosphoric acid,and sulfamic acid. Among them, preferred are acetic acid, lactic acid,propionic acid, formic acid, and sulfamic acid.

Preferably usable as the crosslinking agent are blocked polyisocyanatecompounds and etherified melamine resins. The blocked polyisocyanatecompounds are such that isocyanate groups of a polyisocyanate compoundare completely or partly blocked by a blocking agent. The blocking agentin the blocked polyisocyanate compound is dissociated by heating in thebaking process after the electrodeposition, and the generated isocyanategroups are reacted with functional groups of the cationic base resin,thereby hardening the resin. The etherified melamine resins are obtainedby etherifying melamine with an alcohol such as methanol and butanol.The etherified melamine resin is subjected to a transetherification withthe cationic base resin to promote the crosslinking reaction in thebaking process after the electrodeposition as the blocked polyisocyanatecompound.

Examples of the polyisocyanate compounds used as a material for theblocked polyisocyanate compound include aliphatic diisocyanate compoundssuch as trimethylene diisocyanate, tetramethylene diisocyanate,pentamethylene diisocyanate, hexamethylene diisocyanate, 1,2-propylenediisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate,1,3-butylene diisocyanate, ethylidene diisocyanate, and butylidenediisocyanate; aliphatic, cyclic diisocyanate compounds such as1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate,1,2-cyclohexane diisocyanate, isophorone diisocyanate, and norbornanediisocyanate; aliphatic-aromatic isocyanate compounds such asm-phenylene diisocyanate, p-phenylene diisocyanate, 4,4′-biphenyldiisocyanate, 1,5-naphthalene diisocyanate, 1,4-naphthalenediisocyanate, 4,4′-diphenylmethane diisocyanate, 2,4- or 2,6-toluenediisocyanate or mixtures thereof, 4,4′-toluidine diisocyanate, and1,4-xylene diisocyanate; aromatic dilsocyanate compounds such asdianisidine diisocyanate, 4,4′-diphenyl ether diisocyanate, andchlorodiphenyl diisocyanate; triisocyanate compounds such astriphenylmethane-4,4′,4″-triisocyanate, 1,3,5-triisocyanate benzene, and2,4,6-triisocyanate toluene; tetraisocyanate compounds such as4,4′-diphenyl-dimethylmethane-2,2′,5,5′-tetraisocyanate; and polymerizedpolyisocyanate compounds such as toluene diisocyanate dimer and toluenediisocyanate trimer. Among them, preferred polyisocyanate compounds areisophorone diisocyanate, norbornane dilsocyanate, and4,4′-diphenylmethane diisocyanate.

Examples of the blocking agents for blocking the isocyanate groupsinclude halogenated hydrocarbons such as 1-chloro-2-propanol andethylene chlorohydrin; aliphatic or heterocyclic alcohols such asn-propanol, furfuryl alcohol, and alkyl-substituted furfuryl alcohols;phenol compounds such as phenol, m-creosol, p-nitrophenol,p-chlorophenol, and nonylphenol; oxime compounds such as methyl ethylketoxime, methyl isobutyl ketone oxime, acetone oxime, and cyclohexanoneoxime; active methylene compounds such as acetylacetone, ethylacetoacetate, and diethyl malonate; aliphatic alcohols such asε-caprolactam, methanol, ethanol, and isopropanol; aromatic alcoholssuch as benzyl alcohol; glycol ethers such as ethylene glycol monomethylether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,and diethylene glycol monomethyl ether; etc. Among them, preferred aremethyl ethyl ketoxime and ε-caprolactam.

In the lead-free cationic electrodeposition coating composition, thesolid content ratio of the cationic base resin/the crosslinking agent ispreferably 50/50 to 90/10, more preferably 60/40 to 80/20. When theratio is not within the range of 50/50 to 90/10, the curing efficiencyis often insufficient.

An organic solvent may be used with water in the lead-free cationicelectrodeposition coating composition. Examples of the organic solventsinclude water-miscible organic solvents such as methoxypropanol, ethylcellosolve, propyl cellosolve, butyl cellosolve, 2-ethylhexylcellosolve, n-hexyl cellosolve, methanol, ethanol, isopropyl alcohol,n-butanol, isobutanol, ethylene glycol dimethyl ether, diacetonealcohol, acetone, methyl ethyl ketone, methoxybutanol, dioxane, andethylene glycol monoethyl ether acetate; and water-nonmiscible organicsolvents such as xylene, toluene, methyl isobutyl ketone, hexane, carbontetrachloride, 2-ethylhexanol, isophorone, cyclohexane, and benzene.Among them, preferred organic solvents are butyl cellosolve,2-ethylhexyl cellosolve, and n-hexyl cellosolve, which are excellent infilm forming properties. The amount of the organic solvent is preferably0.1 to 10 parts by mass per 100 parts by mass of the solid contents ofthe cationic base resin and the crosslinking agent.

In the invention, though the monoalkyltin compound is recommended as thedissociation catalyst for dissociating the blocking agent, otherdissociation catalysts may be used. Examples of the other dissociationcatalysts include organic tin compounds such as dibutyltin laurate,dibutyltin oxide, and dioctyltin oxide; -amine compounds such asN-methylmorpholine; and salts of metals such as strontium, cobalt, andcopper. The mass ratio of the dissociation catalyst to the solidcontents of the binder resin is preferably 0.1 to 10% by mass, morepreferably 1.0 to 7.0% by mass. When the mass ratio is less than 0.1% bymass, the catalytic activity is often insufficient. On the other hand,when the mass ratio is increased to more than 10% by mass, the catalyticactivity cannot be correspondingly improved, and further, there is acase where the electrodeposition coating film is decomposed by overbaking.

In addition, crosslinking resin particles, pigments, and variousadditives may be added to the lead-free cationic electrodepositioncoating composition if necessary. Efficiency of maintaining thethickness of the edge portion of the coated article can be improved byadding the crosslinking resin particles. The crosslinking resinparticles may be generated from an acrylic resin, an epoxy resin, aphenol resin, a melamine resin, etc. The crosslinking resin particlesare particularly preferably such as using an acrylic resin from theviewpoint of production easiness. The number average particle size ofthe crosslinking resin particles is preferably 0.02 to 30 μm.

Examples of the pigments used in combination with the aluminum phosphatepigment and the silicon oxide pigment include color pigments such astitanium oxide, iron oxide red, and carbon black; extender pigments suchas aluminum silicate and precipitated barium sulfate; and rustpreventive pigments including phosphomolybdates, which are salts ofphosphomolybdic acid and a di- or tri-valent metal such as aluminum,iron, titanium, zirconium, manganese, cobalt, nickel, copper, zinc, andsilicon. In the case where the lead-free cationic electrodepositioncoating composition contains such a pigment, the composition may furthercontain a resin for dispersing the pigment.

The total amount of the aluminum phosphate pigment, the silicon oxidepigment and the other pigments is 0.2 to 100 parts by mass, preferably0.5 to 50 parts by mass, based on the solid contents of the binderresin.

The lead-free cationic electrodeposition coating composition may beprepared by dispersing the above-mentioned components in an aqueousmedium of the organic solvent and water containing the water-solubleorganic acid or the inorganic acid as the neutralizer.

In the method of the invention, the lead-free cationic electrodepositioncoating composition is applied to the alloyed hot-dip galvanized steelplate. In a preferred embodiment of the method, the lead-free cationicelectrodeposition coating composition is diluted with deionized water tohave the solid concentration of 5 to 40% by mass, preferably 15 to 25%by mass, and the pH value of the composition is adjusted to 5.5 to 8.5,and then the composition is introduced into an electrodeposition bath.The electrodeposition coating is preferably carried out under conditionsof an electrodeposition bath temperature of 20 to 35° C., a coatingvoltage of 100 to 450 V, and a coating time of 1 to 5 minutes. In theprocess of baking the electrodeposition coating film after theelectrodeposition, the substrate temperature is 100 to 250° C.,preferably 140 to 180° C., and the curing time is 5 to 60 minutes,preferably 10 to 30 minutes. The dry thickness of the electrodepositioncoating film is suitably 5 to 40 μm, more preferably 10 to 30 μm. Theabove electrodeposition coating conditions may be controlled to obtainthe dry thickness.

The coated article of the invention is obtained by the method of theinvention, in which the alloyed hot-dip galvanized steel plate is coatedwith the lead-free cationic electrodeposition coating composition. Thecoated article can be suitably used for automobile bodies, automobileparts, etc. A chipping primer coating film and an intermediate coatingfilm may be formed on the electrodeposition coating film, and then, anovercoating film such as a lustering agent-containing film and a clearfilm may be formed thereon, if necessary. Though the electrodepositioncoating film may be baked alone as described above, it may be coatedwet-on-wet with the chipping primer coating film or the intermediatecoating film before curing the composition to simultaneously baking thefilms. The chipping primer coating film, the intermediate coating film,and the overcoating film may be formed by using known compositions underknown conditions for coating outer plates of automobiles, etc.

The invention will be described in more detail below with reference toProduction Examples, Examples, and Comparative Examples. It should benoted that “part(s)” in Examples means “part(s) by mass”.

PRODUCTION EXAMPLE 1

Preparation of Curing Agent

723 parts of isophorone diisocyanate, 333 parts of methyl isobutylketone, and 0.01 part of dibutyltin dilaurate were added to a flaskequipped with a stirrer, a condenser, a nitrogen inlet tube, athermometer, and a dropping funnel, and heated to 70° C. After thecontents were uniformly dissolved, 610 parts of methyl ethyl ketoneoxime was added dropwise over 2 hours, and the reaction was carried outwhile keeping the reaction temperature at 70° C. The infrared spectrumof the reaction mixture was measured during the reaction, and thereaction was continued until absorption of isocyanate groups wasdisappeared, to obtain a curing agent.

PRODUCTION EXAMPLE 2

Preparation of Epoxy-modified Cationic Base Resin

92 parts of 2,4-/2,6-tolylene diisocyanate (mass ratio 8/2), 95 parts ofmethyl isobutyl ketone (MIBK), and 0.5 parts of dibutyltin dilauratewere added to a flask equipped with a stirrer, a condenser, a nitrogeninlet tube, a thermometer, and a dropping funnel, and 21 parts ofmethanol was added thereto dropwise while stirring. The reaction wasstarted at the room temperature, and the temperature of the reactionmixture was raised to 60° C. by heat generation. After 30 minutes of thereaction, 57 parts of ethylene glycol mono-2-ethylhexyl ether was addedto the reaction mixture dropwise from the dropping funnel, and 42 partsof bisphenol A-propylene oxide 5-mol adduct was further added. Thereaction was carried out mainly within the temperature range of 60 to65° C. while measuring the infrared spectrum until absorption ofisocyanate groups was disappeared.

To thus-obtained blocked polyisocyanate was added 365 parts of an epoxyresin having the epoxy equivalent of 188, which was synthesized frombisphenol A and epichlorohydrin, and the temperature of the mixture wasraised to 125° C. Then, 1.0 part of benzyldimethylamine was added to themixture, and reacted at 130° C. until the epoxy equivalent became 410.87 parts of bisphenol A was added to the flask and reacted at 120° C.,and as a result, the epoxy equivalent became 1,190. After cooling theresultant mixture, 11 parts of diethanolamine, 24 parts ofN-methylethanolamine, and 25 parts of a 79% by mass MIBK solution of anaminoethylethanolamine ketimine compound were added to the mixture andreacted at 110° C. for 2 hours. Then, the mixture was diluted with MIBKsuch that the ratio of non-volatile components was 80%, to obtain anepoxy-modified base resin having a cationic group.

PRODUCTION EXAMPLE 3

Preparation of Main Emulsion

672 parts (solid content) of the base resin obtained in ProductionExample 2 and 209.1 parts (solid content) of the curing agent preparedin Production Example 1 were uniformly mixed, and to the resultantmixture was added 3% by mass of ethylene glycol mono-2-ethylhexyl etherbased on the mass of the solid contents. Formic acid was added to theresultant mixture such that the neutralization ratio (the ratio ofneutralizing the cationic groups of the resin) is 41.7%, and 25% by massof a 25% aqueous solution of zinc acetate and ion-exchange water wereadded to dilute the mixture such that the mass ratio of the solidcontents was 30.0% by mass. Then, MIBK was removed under a reducedpressure until the mass ratio of the solid contents became 36.0% bymass, to prepare a main emulsion.

PRODUCTION EXAMPLE 4

Preparation of Pigment Dispersing Varnish

A bisphenol-type epoxy resin having an epoxy equivalent of 450 wasreacted with a 2-ethylhexanol-half-blocked isophorone diisocyanate. Theresultant was converted to a tertiary sulfonium with1-(2-hydroxyethylthio)-2-propanol and dimethylolpropionic acid, toprepare a resin varnish for dispersing pigments having the tertiarysulfonium conversion ratio of 70.6% by mass and the solid resin contentof 60% by mass.

EXAMPLE 1

<Preparation of Lead-free Electrodeposition Coating Composition>

50.0 parts of the resin varnish for dispersing pigments produced inProduction Example 4, 100.0 parts of ion-exchange water, and 100.0 partsof the following granular mixture were dispersed by a sand grindingmill, and further grinded until the particle size became 10 μm or less,to obtain a dispersion paste containing 52.0% by mass of the solidcontents, which contains 40% by mass of monobutyltin oxide and thepigments, and 12% by mass of the solid resin. The pore volume of thesilicon oxide pigment was 1.6 m/g, and the average particle size thereofwas 3.9 μm. The results are shown in Table 1.

TABLE 1 Mass ratio based on Granular mixture Mass ratio total ofpigments Monobutyltin oxide 3 — Aluminum 20 20.6 tripolyphosphateSilicon oxide 20 20.6 Titanium oxide 26 26.8 Carbon black 1 1 Kaolin 3031

Then, 2,000 parts of deionized water, 2,000 parts of the main emulsionof Production Example 3, and 500.0 parts of the dispersion paste weremixed to obtain a lead-free electrodeposition coating composition havingthe solid content of 20.0% by mass. The mass ratio of the monobutyltinoxide to the solid contents of the binder resin was 0.5% by mass, andthe zinc ion content of the composition was 700 ppm.

<Electrodeposition Coating>

A sample of a cold-rolled unprocessed steel plate and a sample of agalvanized steel plate were prepared respectively. Each sample wasdegreased and pretreated with a zinc phosphate-based chemical treatmentagent (trade name SURFDYNE 5000, manufactured by Nippon Paint Co., Ltd.)to be used as a negative electrode, and the above electrodepositioncoating composition was electrodeposited under the conditions of theapplied voltage of 150 to 250 V and the bath temperature of 30° C. toobtain an electrodeposition coating film having the dry thickness of 25μm. The electrodeposition coating film was washed with water and bakedat 170° C. for 20 minutes. The corrosion resistance of eachelectrodeposition coating film was analyzed. The results are shown inTable 2.

TABLE 2 Comparative Comparative Comparative Items Example 1 Example 2Example 3 Example 4 Example 5 Example 1 Example 2 Example 3 Monobutyltinoxide 1.0 0.0 1.0 1.0 1.0 1.0 1.0 1.0 Mass ratio Aluminumtripolyphosphate 20.6 20.6 5.2 30.0 15.4 30.0 0.0 41.2 based on Siliconoxide 20.6 20.6 30.0 5.2 15.7 0.0 30.9 41.2 total of Titanium oxide 26.826.8 26.8 26.8 26.8 26.8 26.8 16.6 pigments Carbon black 1.0 1.0 1.0 1.01.0 1.0 1.0 1.0 Kaolin 31.0 31.0 37.0 37.0 41.1 42.2 41.3 0.0 Zinc ioncontent (ppm) 700 700 680 720 700 720 640 730 Evaluation Smoothness (Ra;μm) 0.18 0.23 0.18 0.19 0.20 0.19 0.19 0.41 Corrosion resistance test(cold- 1.5 1.7 1.4 1.4 1.5 1.7 2.4 1.9 rolled steel; cut portion)Corrosion resistance test (cold- 1.3 1.4 1.2 1.2 1.2 1.5 2.0 1.6 rolledsteel; edge portion) Corrosion resistance test 2.8 3.0 3.1 3.1 2.9 4.84.8 3.3 (galvanized steel; cut portion) Corrosion resistance test 2.52.7 2.7 2.8 2.7 4.5 3.7 3.0 (galvanized steel; edge portion) The amountof monobutyltin oxide is shown as the mass ratio (% by mass) to thesolid contents of the binder resin; the amount of each pigment is shownas the mass percentage; and the zinc ion content (ppm) is a zinccompound concentration of a supernatant liquid obtained by centrifugingeach composition for 1 hour at 12,000 round per minute.<Smoothness Test>

The surface roughness (Ra) of the coating film was measured inaccordance with JIS-B0601.

<Corrosion Resistance Test>

The obtained coated article was subjected to a corrosion test containing16 hours of salt spray, 2 hours of drying, 4 hours of dipping in a saltwater, and 2 hours of drying. The corrosion test was repeated 15 times,and the swell width of the one side of the cut portion was measured.

EXAMPLES 2 TO 6 AND COMPARATIVE EXAMPLE 1 TO 3

Lead-free electrodeposition coating compositions were prepared,electrodeposited to samples, and evaluated with respect to corrosionresistance, in the same manner as Example 1 except for changing theamounts of the aluminum tripolyphosphate pigment, the silicon oxidepigment, and the other pigments. The results are shown in Table 2.

It was clear from the above results that the lead-free cationicelectrodeposition coating compositions containing a particular amount ofthe aluminum tripolyphosphate pigment and a particular amount of thesilicon oxide pigment showed improved corrosion resistance withoutreduction of the smoothness in the case of using the cold-rolled steelplate or the galvanized steel plate.

In the method of the present invention, the alloyed hot-dip galvanizedsteel plate is coated with the lead-free cationic electrodepositioncoating composition comprising the aluminum phosphate pigment and thesilicon oxide pigment with the particular shape, whereby thus-obtainedcoated article of the invention has edge portions and general surfacesexcellent in the corrosion resistance. Further, in the case of using thelead-free cationic electrodeposition coating composition containing themonoalkyltin compound, the formed coating film is excellent in adhesionto an upper coating film and hardly deteriorates the surface of theupper coating film to show excellent film performances. Furthermore, thenonvolatile catalyst is preferable in view of environment and industrialhealth. The coated article of the invention has the abovecharacteristics, and thereby can be suitably used for products requiringhigh corrosion resistance such as automobile bodies, automobile parts,and home electric appliances.

1. A method for coating an alloyed hot-dip galvanized steel plate with a cationic electrodeposition coating composition in which the lead content thereof is less than or equal to 10 ppm, and which includes a binder resin and a plurality of pigments, the method comprising: electrodepositing the lead-free cationic electrodeposition coating composition onto the alloyed hot-dip galvanized steel plate, wherein the binder resin includes a cationic base resin and a crosslinking agent, and the pigments include 5 to 30% by mass of pigment formed from a compound having an aluminum-containing cation and a phosphate-containing cation, and 5 to 30% by mass of a silicon oxide pigment having a pore volume of 0.44 to 1.8 ml/g and an average particle size of approximately 10 μm or less.
 2. The method according to claim 1, wherein the cationic electrodeposition coating composition comprises 0.1 to 10% by mass of a monoalkyltin compound based on a mass of solid contents of the binder resin.
 3. A coated article, comprising: an alloyed hot-dip galvanized steel plate coated with a cationic electrodeposition coating composition in which the lead content thereof is less than or equal to 10 ppm which is electrodeposited on the steel plate, the cationic electrodeposition coating composition including a binder resin and a plurality of pigments, wherein the binder resin includes a cationic base resin and a crosslinking agent, and the pigments include 5 to 30% by mass of pigment formed from a compound having an aluminum-containing cation and a phosphate-containing cation, and 5 to 30% by mass of a silicon oxide pigment having a pore volume of 0.44 to 1.8 ml/g and an average particle size of 10 μm or less.
 4. The coated article according to claim 3, wherein the cationic electrodeposition coating composition comprises 0.1 to 10% by mass of a monoalkyltin compound based on the a mass of the solid contents of the binder resin.
 5. An automobile body, comprising: an alloyed hot-dip galvanized steel plate coated with a cationic electrodeposition coating composition in which the lead content thereof is less than or equal to 10 ppm which is electrodeposited on the steel plate, the cationic electrodeposition coating composition including a binder resin and a plurality of pigments, wherein the binder resin includes a cationic base resin and a crosslinking agent, and the pigments include 5 to 30% by mass of pigment formed from a compound having an aluminum-containing cation and a phosphate-containing cation, and 5 to 30% by mass of a silicon oxide pigment having a pore volume of 0.44 to 1.8 ml/g and an average particle size of 10 μm or less.
 6. The automobile body according to claim 5, wherein the cationic electrodeposition coating composition comprises 0.1 to 10% by mass of a monoalkyltin compound based on the a mass of the solid contents of the binder resin. 